First of all, that's not how queuing theory works.
Second of all, (after a quick search):
Capacity of Superchargers Using an Erlang-B Model Even a simple of doubling of stalls at a supercharger increases the capacity by about 3x. It's not linear.
Third of all, assuming the model 3 is released on time (late 2017, early 2018), you think they're going to be able to ramp up production from 0 to 500k units in 2 years? Even with the GF up and running? I'm not convinced.
And lastly, Tesla added 114 superchargers in 2015 in the US, for a total of 265 by the end of 2015. Even if we assume the ramp is linear (it likely wont be once the Model 3s start being produced) in 4 years, the number of stalls would have roughly tripped from where are now. That means people are going to have more options of places to stop. Think gas stations. You see one with a long wait, look at your fuel gauge, eh, I can make it to the next one, and you go. If you tripple the number of superchargers, sure, they wont be as ubiquitous as gas stations, but it will give people more options as to where to stop.